The use of PAUT for the inspection of pipeline girth weld has been described in various publications such as “Pipeline Girth Weld Inspection using Ultrasonic Phased Arrays” (by Michael Moles, Noel Dube, Simon Labbé, Ed Ginzel). The practice is also incorporated in industrial standard practice such as ASTM E-1961-11 “Standard Practice for Mechanized Ultrasonic Testing of Girth Welds Using Zonal Discrimination with Focused Search Units”.
Practically, this inspection is based on the zone discrimination technique which involves the definition of and calibration of multiple beams to obtain desired detection performance on a set of pre-defined artificial defects. The PAUT beams use a pitch-catch configuration which means each beam is impacted by the definition of a transmitter and a receiver. The calibration itself is then relatively complex and time consuming because of the large number of possible settings for each beam.
Furthermore, since the required configuration is dependent on the specific weld geometry to be inspected, a new calibration is required every time changes are made to the inspection system. For calibration of pitch-catch inspection with the zone discrimination technique, an inspector needs to calibrate for each zone of interest in a calibration block to be sure to meet inspection criteria for all potential defects. More specifically, for each zone, the inspector needs to align the probe on the relevant calibration reflector, to adjust the steering angle, the aperture (position and size) and focalization of the transmitter, the aperture of the receiver (position and size), focalization and the gain of the receiver to perform calibration. With today's methods, due to the plurality of calibration reflectors, a typical calibration procedure takes several hours to perform and requires a high degree of expertise by the operator.
A side effect of the current level of complexity of the calibration is a compromise in calibration requirement between realistic calibration time and performance.
PAUT Girth weld inspection is the most common example of the use of pitch-catch PAUT, but it must be understood that the same or similar limitations are found in most systems that rely on pitch-catch PAUT.
So, there is therefore a need for a method to automate the calibration process of pitch-catch PAUT inspection in order to reduce the calibration time and the dependency on inspector's skills.
There's also a need for a method that would meet tighter calibration tolerance than current PAUT inspection methods in order to improve detection performances and reproducibility.